Low-Level Parasite Persistence Drives Vasculitis and Myositis in Skeletal Muscle of Mice Chronically Infected with Trypanosoma cruzi.

In chronic Trypanosoma cruzi infection, the cause of Chagas disease, life-threatening inflammatory diseases develop over time in the heart, esophagus, and colon of some patients. C57BL/6 mice infected with the myotropic Colombiana strain of T. cruzi model many of the immunological and parasitological features of human infection but succumb to chronic paralyzing myositis and skeletal muscle vasculitis, not cardiomyopathy or gastrointestinal disease. Here we show that T cell depletion in the chronic phase of this model increased tissue parasitism to acute-phase levels and induced neutrophilic skeletal muscle inflammation. Conversely, after daily treatment with the trypanocide benznidazole for 8 weeks during the chronic phase, viable parasites were no longer detectable, myositis completely resolved, vasculitis was ∼80% reduced, fibrosis was reduced, and myofiber morphology normalized. After the drug was discontinued, parasitism rebounded, and immunopathology recurred. The parasite load was statistically strongly correlated with the severity of inflammation. Thus, both T cell immunity and trypanocidal pharmacotherapy suppress to very low levels, but do not cure, T. cruzi infection, which is necessary and possibly sufficient to induce crippling chronic skeletal muscle myositis and vasculitis in the model.